Ethan asserts that I am not a physical cosmologist. I wasn't aware that he was the only person designated to bestow that title. I certainly am an active scientist who works on cosmology, among other topics. I am a full professor at a research university, achieving this rank first at the University of Maryland before moving to Case Western. That means, among other accomplishments, that I had to receive glowing letters of recommendation from other scientists, including some very eminent physical cosmologists.
Ethan, on the other hand, is a blogger. He seems to be fond of overstating his qualifications. Reading his blog, he seems to have a reasonable understanding of modern cosmology, at the level I would expect of any student who took my graduate course on the subject. His knowledge of galaxy dynamics, the subject here, does not appear to be at that level.
Check for yourself. NASA supports the Astrophysical Data System, which allows you to search the scientific literature on this subject: http://adsabs.harvard.edu/abstract_service.html. If you type my name into the author field and restrict the search to refereed papers, you will see 74 publications. If you then sort by citations, you will see over 6,000. That is the record of an active scientist who is well regarded in the field. If instead you enter "siegel, e." you will see rather less.
I don't claim to know the right answer here. We have yet to detect dark matter particles in the laboratory, so we can hardly claim to be sure they exist. MOND, strange as it is, does have some of its predictions come true. I find both possibilities fascinating.